1. Field of the Invention
The present invention relates to a process for preparing ferromagnetic chromium dioxide.
2. The Prior Art
Ferromagnetic chromium dioxide products are, of course, known. Such products are used in various aspects of magnetic recording, such as audio and video magnetic recording tapes, tapes and memories for electronic computers, magnetic discs and cards, etc.
In some of these uses, for example, video recording tapes, computer tapes and high fidelity audio recording tapes, it is necessary that the chromium dioxde possess particular magnetic characteristics, that is, a coercive force of at least 300 Oersteds, a magnetization saturation that is as high as possible and a ratio of residual mangetizaton to saturation magnetization of at least 0.4. Moreover the chromium dioxide must consist of elongated particles with as uniform a length as possible.
There are known ferromagnetic chromium dioxides that are obtained by thermally decomposing CrO.sub.3 at high temperatures and under high pressures, the CrO.sub.2 thereby obtained is a pure product, that is, it is free of other chromium oxides, but it possesses magnetic and granulometric characteristics which are insufficient to permit its use in the above mentioned areas.
Specifically, its coercive force is less than 200 Oersteds and its granulometric characteristics are rather poor. In fact the obtained particles are rather large, non-homogeneous and not very elongated.
A chromium dioxide of smaller and more elongated particle size and characterized by higher values of coercive force is, on the contrary, obtained by heating the chromium anhydride under pressure and in the presence of particular modifying elements or of compounds thereof, such as antimony or tin. In this chromium dioxide, the modifying elements are inserted into the crystalline lattice of the chromium dioxide. However, in such products it is difficult to balance the necessity of obtaining high coercive forces with the necessity of getting high values of magnetization and thus, the product is unsuitable for applications where both high coercive force and a high magnetization value are required.
Copending application Ser. No. 231,028, filed Mar. 1, 1972 and owned by unrecorded assignee hereof, describes a process for the preparation of chromium dioxide starting from a hydrated trivalent chromium chromate of the formula: EQU Cr.sub.2 (CrO.sub.4).sub.3.nH.sub.2 O
wherein n varies from 1 to 8. The chromium chromate hydrate is decomposed into CrO.sub.2 at temperatures between 250.degree. and 500.degree.C. and under pressures between 30 and 1,000 atm.
The product thereby obtained has good magnetic and granulometric characteristics. The maximum coercive force of that product is about 350 Oersteds, while the particles are elongated and of very uniform dimensions. The best results according to that process are obtained when n is between 2 and 4. When n is greater than 4, the magnetic properties tend to become less good as the value of n rises. When n is greater than 8, the obtained product is in general impure due to the presence of CrOOH, which forms as a separate phase or coats the CrO.sub.2 particles thereby reducing both the residual magnetization and the saturation magnetization.
U.S. Pat. No. 3,874,923, relates to ferromagnetic compositions based on CrO.sub.2 modified with lanthanum, yttrium, barium or strontium. The addition of these modifiers, in elemental form or in the form of compounds thereof allows one to obtain, when using the same process as described in copending application Ser. No. 231,028 products with an even greater coercive force, (i.e., up to about 440 Oersteds). In this process as well, the optimum value of n in the formula Cr.sub.2(CrO.sub.4).sub.3.nH.sub.2 O is between 2 and 4. When n is greater than 8, the product is generally impure due to the presence of CrOOH.
The products obtained according to U.S. Pat. No. 3,874,923, particularly when lanthanum is used as the modifier, are very well suited for use in high fidelity audio tapes and in video tapes employed in coventional equipment. However, in the case of video tapes used in equipment requiring particularly high performance (for example in "helical scan" recordings) even higher coercive forces of around 500 Oersteds and more are desirable. Materials having this property are provided by the present invention.